I'm not going to beat a dead horse and resummarize the series - the first review on here already did a wonderful job of that. Clannad is a rare diversion from the typical romance/high school type anime in that it stays interesting throughout and does not dwell on one single part of the plot too much. The animation looks nice and the characters are very cute and likeable. I really like Nagisa's parents - hilarious!
Most of the themes here are not particularly deep but it is also not one of those anime that suffers from being just plain goofy. There are, of course, the usual slapstick jokes mixed in (girl who beats up the boys and threatens people, girl with a strange obsession with something, etc) but it's not too overdone. There are parts of the series that are surprisingly emotional. The whole Fuko plotline definitely made my eyes a little watery at times! Once that part of the story was finished there were little throwbacks to it occasionally, but they did not dwell on it for any long period of time. The main story focuses on the creation of the drama club and climaxes with the drama club's very first performance. I was not particularly fond of the last episode of Season 2 "Another World: Tomoya Story."
However, this episode is more of an alternate ending and not really part of the plot. The original Season 2 ending is very romantic and adorable! I have not watched After Story yet - this is the last installment of the series. However, I was very happy with the (real) ending to Season 2.
The series as a whole is fun, well paced, and worth a watch.